Water is the single most important resource when it comes to running your workshop in My Time at Sandrock. If one machine runs out of fuel, that machine shuts down. However, if you run out of H2O, everything shuts down.
Unfortunately, it isn’t exactly plentiful in the desert community of Sandrock. There’s only one source of it, and that’s the oasis. You can’t exactly just dunk a bucket in and walk off. You’re going to have to pay for it. And to do so, you’ll need to swing by Water World.
If you’re walking from your workshop, Water World is located on the far side of the oasis on the boardwalk. It’s just a cash register overlooking the oasis. Interact with that, and you’ll have the option to purchase large quantities of water (among other things). Once you have the liquid, you can take it back to the tank on the side of your workshop and fill it up using the “adjust” button on the right side of the screen.
That’s not the only way to get water; it’s just the quickest and easiest way. The other way is to collect dew by bashing plants or building collectors. You can then process that into water. It’s a great way to save money, but building the infrastructure to handle it is a bit out of your reach at the beginning of the game. Until you have the space and technology, it’s best just to visit Water World.